Quantitative separation of monomeric U(IV) from UO2 in products of U(VI) reduction.

The reduction of soluble hexavalent uranium to tetravalent uranium can be catalyzed by bacteria and minerals. The end-product of this reduction is often the mineral uraninite, which was long assumed to be the only product of U(VI) reduction. However, recent studies report the formation of other species including an adsorbed U(IV) species, operationally referred to as monomeric U(IV). The discovery of monomeric U(IV) is important because the species is likely to be more labile and more susceptible to reoxidation than uraninite. Because there is a need to distinguish between these two U(IV) species, we propose here a wet chemical method of differentiating monomeric U(IV) from uraninite in environmental samples. To calibrate the method, U(IV) was extracted from known mixtures of uraninite and monomeric U(IV) and tested using X-ray absorption spectroscopy (XAS). Monomeric U(IV) was efficiently removed from biomass and Fe(II)-bearing phases by bicarbonate extraction, without affecting uraninite stability. After confirming that the method effectively separates monomeric U(IV) and uraninite, it is further evaluated for a system containing those reduced U species and adsorbed U(VI). The method provides a rapid complement, and in some cases alternative, to XAS analyses for quantifying monomeric U(IV), uraninite, and adsorbed U(VI) species in environmental samples.

Oxidative Dissolution of Biogenic Uraninite in Groundwater at Old Rifle, CO.

Reductive bioremediation is currently being explored as a possible strategy for uranium-contaminated aquifers such as the Old Rifle site (Colorado). The stability of U(IV) phases under oxidizing conditions is key to the performance of this procedure. An in situ method was developed to study oxidative dissolution of biogenic uraninite (UO2), a desirable U(VI) bioreduction product, in the Old Rifle, CO, aquifer under different variable oxygen conditions. Overall uranium loss rates were 50-100 times slower than laboratory rates. After accounting for molecular diffusion through the sample holders, a reactive transport model using laboratory dissolution rates was able to predict overall uranium loss. The presence of biomass further retarded diffusion and oxidation rates. These results confirm the importance of diffusion in controlling in-aquifer U(IV) oxidation rates. Upon retrieval, uraninite was found to be free of U(VI), indicating dissolution occurred via oxidation and removal of surface atoms. Interaction of groundwater solutes such as Ca²âº or silicate with uraninite surfaces also may retard in-aquifer U loss rates. These results indicate that the prolonged stability of U(IV) species in aquifers is strongly influenced by permeability, the presence of bacterial cells and cell exudates, and groundwater geochemistry.

Electron microbeam investigation of uranium-contaminated soils from Oak Ridge, TN, USA.

Two samples of uranium-contaminated soil from the Department of Energy's Oak Ridge Reservation in Oak Ridge, Tennessee were investigated using electron microprobe analysis and transmission electron microscopy. The objectives of this research were to identify and characterize soil particles and rock chips with high uranium concentrations, to investigate the extent of uranium penetration into chips of parent material, and to identify solid-phase hosts for uranium in the samples. Three distinct solid-phase hosts for uranium have been identified: (1) iron oxyhydroxides, including goethite and ferrihydrite; (2) mixed Mn-Fe oxides; and (3) discrete uranium phosphates. In all three, uranium is associated with phosphorus. The ubiquitous U-P association highlights the influence of phosphate on the environmental fate of uranium. Uranium-bearing phases are found well within chips of weathered shale, as far as 900 microm from fractures and chip edges, indicating that uranium has diffused into the shale matrix.